In order to comply with Federal Communications Commission Regulations and to prevent interference with other radiotelephones, a radiotelephone, such as a cellular, portable, or mobile radiotelephone device, requires a high precision crystal oscillator. Many current radiotelephones use an AT-cut crystal for the crystal oscillator. An AT-cut crystal has the most stable frequency over temperature.
Some devices use temperature compensation to further increase the frequency stability of the crystal oscillator over the desired temperature range. In these devices a temperature-dependent network is used to influence the frequency of the oscillator. For example, a temperature measuring device is used to determine the temperature and adjust the voltage applied to a tuning device, such as a varactor, which is connected to the crystal. Buffering is used to isolate the oscillator from the effects of its environment and connected load, such as the stage to which the oscillator is connected. Such a device is frequently referred to as a Temperature Compensated Crystal Oscillator (TCXO).
To further increase the accuracy of the oscillator, an automatic frequency control (AFC) loop is sometimes added to a TCXO. This type of device is frequently referred to as a Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO). This device requires two inputs to the oscillator circuit: one input for the temperature sensing circuit, and one input for the AFC. The TCXO tracks the frequency of the external reference as long as the external reference is available. However, if the external reference disappears or even momentarily drops out, then the VCTCXO will lose its tracking and will rely only upon temperature compensation. However, if the aging is sufficiently severe and the channel spacing is sufficiently narrow, the VCTCXO may not stay tuned to the correct channel.
Another type of high precision crystal oscillator is the Digitally-Compensated Crystal Oscillator (DCXO). This device uses a dedicated controller whose sole function is to compensate for the changes in crystal frequency due to temperature. The manufacturer "trains" the dedicated controller for each DCXO by measuring the frequency of oscillation of the crystal of that DCXO at different temperatures and providing the dedicated controller with a correction table which specifies the amount of offset needed for each given temperature. This oscillator requires only one input but is expensive because a dedicated controller is required for each crystal and, further, each crystal must be individually calibrated in order to generate a correction table so that the dedicated controller will be able to compensate for the temperature characteristics of that particular crystal.
However, crystals are not subject only to the effects of temperature. They are also subject to the effects of aging. As a crystal ages the frequency of oscillation changes. This change is in addition to any change that is caused by temperature. Therefore, after a while, the prior art frequency-temperature correction table is no longer accurate. None of the prior art devices compensate for aging. These devices merely compensate for temperature variations.
This aging does have an effect on the VCTCXO if the VCTCXO does not have a frequency reference which it can track. That is, until the VCTCXO receives a reference signal, the VCTCXO can only compensate for temperature changes because it does not have a reference signal which it can track. This can cause the radiotelephone to operate outside of the allowable tolerance established by Federal Communications Commission (FCC) or other appropriate governing body regulations. If the channel spacing is very narrow and the aging is severe the radiotelephone may listen for instructions from the Mobile Telephone Switching Office (MTSO) on the wrong channel, thereby preventing the radiotelephone from establishing communications with the MTSO. In this case the radiotelephone will appear to be inoperative.
Some manufacturers attempt to compensate for aging by pre-aging (burning-in) the crystals. However, this increases the cost of the crystals, and only compensates for the initial aging of the crystal (infant aging) and does not compensate for long-term or cumulative aging.
Therefore, there is a need for an oscillator which is temperature stable and which is not dependent upon a reference signal for its accuracy.
There is also a need for an oscillator which has compensation for the effects of aging.
There is also a need for an oscillator which can use an AT-cut crystal which is not pre-aged.
There is also a need for a radiotelephone which uses a low cost oscillator circuit.
There is also a need for an oscillator which has compensation for the effects of temperature without using a dedicated microcontroller.
There is also a need for an oscillator which does not require temperature training and/or testing.
There is also a need for a radiotelephone which maintains minimum frequency error, even when an external reference is temporarily not available.